Since I'm doing a lot of weapon crafting that generates junk mods at Starlight DriveIn, I decided it would be practical to set up a Trader stall there to sell my mods directly instead of having to go to the Castle to sell them.

But so far, the Settler I designated as Trader doesn't appear to be working.

Whenever I try to interact, all I get are the standard Trade Item responses.

There is no dialogue about selling/buying.

Do you need to find designated Traders and send them to these stalls?

That could be problematic, because recruitable Traders are not very common.

And in some cases, I don't want to take specific traders away from their homes (Holt and Vault 81, for example).

So far all I have are VaultTec Rep and Trader Rylee, both of whom have been assigned to specific settlements.

Oddly, making a Doctor is easy by comparison, I just pick someone and send them to the stall, the Doctor dialogue immediately takes over.

The settlement merchants work just like regular merchants as far as their shop inventory goes. You can't modify it or steal it (without mods or console commands). They'll regenerate their shop inventory every few days. You can't give them stuff to sell. They seem to scrounge it up themselves.

The 'Barter' option is the buy/sell option and uses their shop inventory.

The 'Trade' option is the give/take option and uses their personal inventory, just like regular settlers.

As far as the settlement mechanics go... settlers assigned to shops will man them during most of the daytime hours, then go about other stuff at night (such as attending a bar or sleeping). Occasionally they'll take a break or take a day off for no apparent reason (bug maybe?). Generally you can only barter with them during shop hours when they are near the shop.

Some of the merchants stalls are bugged and will never be manned. You can still barter if you force the NPC near the stall during shop hours, but they won't stand there. Weapon Tier 1 and Armor Tier 1 seem to be bugged in this manner. All the tier 3 shops work.

The Food/Drink 'Bar' stand acts as a meeting point for your settlers at night. All the settlers in town and other NPCs (like your companions) will gather there at night before bedtiem for some drinking. Sometimes people randomly visit during the day to eat noodles. If there are chairs nearby, NPCs will often sit down. You can setup a really cool bar or restaurant this way.